The flow dynamics of gases entering and exiting internal combustion engines is one of the controlling factors of engine performance. Most engines must work over a wide speed and load range, making it difficult to achieve optimum efficiency over more than a narrow part of that range. For simplicity, economy and durability, most conventional four stroke engines use the tried-and-true fixed camshaft systems that have constant phase (when the valves are opened), duration (how long the valves are held open) and lift (how far the valves are lifted off their seats). This leads to certain design compromises to achieve acceptable performance. An engine that produces high torque for its capacity at low engine speeds usually gives poor torque at higher engine speeds, and vice versa. In a paper given at the Society of Automotive Engineers Congress in Detroit (Hara, Kumagai and Matsumoto, 1989, SAE paper 890681), the authors present experimental results on an engine in which the timing and lift were varied. Torque was improved by 7% at 1600 rpm by variation of lift, and the improvement at 6000 rpm was 14%. Alteration of lift of the intake valve produced most of the effects seen.
Many approaches have been proposed and tried in attempts to optimize the flow processes. Improvements to the flow dynamics are achieved by three separate but interrelated processes: variable phase, variable duration, and variable lift. It is well known that engines that produce high torque at low speeds have lower overlap between the closing of the exhaust valve and the opening of the intake valve. Small overlap allows for little communication between the exhaust gases and the incoming fresh charge, limiting the amount of uncontrolled mixing. This leads to stable operation. However, at high speeds the inertia of the gases requires a greater period of overlap to allow for gas exchange. The simplest way of achieving the change in overlap is to alter the relative timing, or phasing, of the intake camshaft to the crankshaft and exhaust camshaft.
If the phase of a valve event is altered, say advancing the valve opening to an earlier crankshaft angle, then the closure of that valve is also advanced. In many cases this causes a reduction of the amount of combustible gas that can enter the engine. To overcome this situation, the duration of the valve event may be altered. In the example above, as the engine speed is increased and the valve overlap is increased (opening the intake valve earlier), the period that the intake valve stays open is extended to delay the closing.
The peak lift of valves is designed to accommodate gas flow at maximum engine speeds without significant pressure drops. This is more important for the intake process than the exhaust process, where the piston pushes the gases out. At engine speeds below maximum, the velocity of incoming gases through the valve curtain will produce less turbulence, and may lead to lower torque than would be achieved with a smaller valve opening. By varying valve lift with engine speed, torque may be enhanced over the entire operating range of the engine. Additionally, reduced valve lifts at lower speeds may reduce the frictional losses of the valve train, depending on the design.
There are many examples in the U.S. patent literature of methods of varying either or all of phase, duration and lift. Many authors have recognized that engine performance over a wide speed range may be improved by providing a means of switching between two independent cam profiles for low and high speed operation. Such an "on or off" type controller will provide different values of phase, duration and lift between the two (or possibly more) different engine speed ranges, resulting in improved performance and efficiency for each speed range. However, within each speed range, there is no means of varying phase, duration and/or lift. Examples of such mechanisms are given in U.S. Pat. No. 2,934,052 by Longenecker, U.S. Pat. No. 4,151,817 by Mueller, U.S. Pat. No. 4,205,634 by Tourtelot, U.S. Pat. No. 4,970,997 by Inoue, et al. and U.S. Pat. No. 5,113,813 by Rosa. In SAE paper 890675 (Inoue, Nagahiro, Ajiki and Kishi, 1989) the authors point out that the variable valving system described in U.S. Pat. No. 4,970,997 would have greater mass than conventional systems. Extensive redesign of each component was undertaken to reduce this mass.
Another means of achieving variation in all three parameters is to use an axially moveable camshaft, with a variable profile in the axial direction. In this case there may be a smooth transition between different values of phase, duration and lift, although the relationship between all three is again fixed for a particular axial position of the camshaft. U.S. Pat. No. 3,618,574 by Miller and U.S. Pat. No. 5,080,055 by Komatsu, et al., describe such devices.
An alternative approach to varying all three parameters involves the use of multi-part rocker arms, with one or more of the arms pivoted eccentrically. In U.S. Pat. No. 4,297,270 by Aoyama two interacting rocker arms function to vary phase, duration and lift. In U.S. Pat. No. 4,438,736 by Hara, et al., problems with adjustment clearance and noise in the aforementioned patent are considered to be unacceptable. In this patent, as well as U.S. Pat. No. 4,498,432 by Hara, et al., the problem of clearance and noise is addressed by using an extendible hydraulic cam follower. In all of these cases, the phase, duration and lift of the valves is somewhat inflexible. These systems will probably experience higher levels of friction than conventional systems.
In U.S. Pat. No. 4,714,057 by Wichart, the author discloses control over all three parameters by using a multi-part rocker arm, and a control cam as well as the lift cam. A major purpose of their invention is to be able to control engine load without a throttle plate. Friction may be a problem with this design.
An innovative scheme is disclosed in U.S. Pat. No. 4,898,130 by Parsons, to vary the phase, duration and lift of the valves, with an eccentrically mounted oscillating drive. Besides giving good control over all three parameters, the mechanism disposes of the main valve spring, aiding in lowering friction. The technology is radically different from that employed in current engines, however, and requires the use of a rather long pushrod.
There are several different means disclosed for varying the lift only of valves. In U.S. Pat. No. 5,119,773 by Schon, et al., there is interposed either a slidable or pivoted member between the camshaft and the valve, with a movable pivot providing control for its movement. The mechanisms described appear to have higher friction loads than conventional valve gear, as well as high lateral forces and increased reciprocating mass.
In U.S. Pat. No. 4,187,180 by Buehner and U.S. Pat. No. 4,519,345 by Walter, valve lift only is varied by moving the point of application of the lifting member to the rocker arm. In each case, the mechanism is applied to a pushrod engine, and appears unsuitable for an overhead camshaft geometry. The design does retain conventional valve clearance adjustment.
Movement of the rocker arm pivot is favored in U.S. Pat. No. 4,986,227 by Dewey. In this approach, the rocker arm has an arcuate upper surface upon which rides a movable bearing held by a lever arm, with the lower end of said lever arm being pivoted in the head. Lateral location of the rocker arm is required to ensure the arm remains in contact with both the camshaft and the valve top. This is achieved by a special cap atop the valve, and a suitable recess in the end of the rocker arm. Adjustment of valve clearance differs from conventional valve trains. A similar rocker arm retention scheme is used by Schneider in U.S. Pat. No. 5,189,997, for an overhead cam engine with a finger follower arrangement.
Variable valve lift is achieved by yet another means in U.S. Pat. No. 5,031,584 by Frost. Two fixed pivot rocker arms are combined with a movable interposed member to alter the mechanical advantage of the camshaft to valve movement. The design appears complex, and subject to higher friction losses than conventional designs. Another means of achieving variable valve lift by moving the pivot point is given by Hoffman in U.S. Pat. No. 5,205,247. A rotatable pivot shaft locates a pivot point for a circular rocker arm. The centers of the circular arms of the rocker arm are located on the same side of the rocker arm as the pivot. As the pivot point is varied, the circular shape of the rocker arm offers the same geometry to the cam and valve at each location of the pivot. The valve timing is altered by using different radii and/or offset centers for the arc segments either side of the pivot point, combined with cam profiles that differ from standard profiles.
Entzminger offers a simple concept for varying valve lift in U.S. Pat. No. 4,721,007. A toothed pivot shaft mates with a toothed rack embedded in an elongate rectangular slot in the rocker arm. The pivot shaft translates and rotates simultaneously, following a linear path defined by another stationary toothed rack. This approach has the advantage that the cam end and valve end of the rocker arm resemble a conventional rocker arm. The potential disadvantages of this design are the size of the pivot shaft and the rocker arm, and the flexibility of the rocker arm.
Another class of actuation mechanisms that can vary lift and duration is that of hydraulic actuation, with lost motion. In this method, the cam follower allows enclosed hydraulic fluid to leak out either through a fixed orifice, or through a controlled orifice. For the passive mechanism, the result is that the valve will not open as far or as long at low engine speeds, while at high speeds the leakage is insufficient to significantly alter the valve movement from a conventional system. The active control approach allows lift and duration to be controlled more closely. The result is that conventional throttling may be discarded, as valve motion may be enough alone to control the intake charge. Such a system is described in SAE paper 930820 (Urata, et al., 1993). The drawbacks to the system include non-recovery of the work of opening the valve, variations in motion as the oil changes viscosity with temperature, and complexity. An engine equipped with this system showed significant improvement in torque at lower engine speeds, and when installed in a vehicle, exhibited a fuel economy gain of 7%.